Cam mechanisms

ABSTRACT

The present invention provides a cam mechanism for controlling the motion of cylinder head valve means of an internal combustion engine, the cam mechanism comprising: a camshaft having first and second cams which rotate therewith, the first and second cams having different cam profiles, and a drive mechanism operable to relay drive from the first or the second cam to the controlled cylinder head valve means, the drive mechanism comprising first and second cam follower means engagable respectively with the first and second cams and engagement means for engaging the first and second cam follower means with their respective cams. The drive mechanism has three operating conditions: a first operating condition in which the first cam follower means is engaged with the first cam and the valve means is driven by the first cam; a second operating condition in which the second cam follower means is engaged with the second cam and the valve means is driven by the second cam; and a third operating condition in which both the first and second cam follower means are disengaged from the first and second cams.

The invention relates to a cam mechanism for the cylinder head valves ofan internal combustion engine.

Cam mechanisms are well known. In conventional cam mechanisms a camshaftis provided with several cam lobes and the cylinder head valves of theengine are each driven by one of the cam lobes of the camshaft. In suchconventional engines, the cylinder head valves are each driven by onecam profile only for all engine speeds and loads. This isdisadvantageous since a cam profile best suited for high speed, highload operation of an internal combustion engine is not suited for lowspeed, low load operation and vice versa.

There have been a number of proposals for cam mechanisms which enable avalve to be driven by two different cam profiles, a first cam profile atlow engine speeds and loads and second cam profile at higher enginespeeds and loads. One example of such a system is described inWO-A-91/12415. In this published application a cam mechanism is shownwhich has a camshaft with first and second cam lobes mounted thereon forrotation therewith, the first and second cam lobes having different camprofiles. First and second finger followers are provided with first andsecond follower means respectively arranged to follow the profiles ofthe first and second cams. The first and second finger followers arerockable about fulcrum means comprising a fixed fulcrum in the form of arocker shaft and a movable fulcrum. The first cam and the first followermeans of the first finger follower together form a first short durationcam mechanism for low speeds of the engine, the movable fulcrum being ina first position at low speeds in which the second finger follower isout of engagement with the second cam. At higher speeds the movablefulcrum is moved to a position in which the second finger followerbrings a second cam follower into abutment with the second high lift camand the controlled valve is moved under the action of the cam of higherlift, the second cam.

In one embodiment shown in the prior art document WO-A-91/12415 two sideby side valves are controlled by a cam mechanism which comprises twofinger followers which sandwich a third finger follower. The two outerfinger followers engage cam profiles for low speeds of the engine, thecam profiles being both low lift profiles, but the profiles beingdifferent to each other. At high speeds, the third inner finger followerengages a high lift cam which has a lift higher than both of the othertwo cams and thus both valves are controlled by the lift of the highlift cam.

The prior art specification WO-A-91/12415 teaches that the low lift camcan be replaced by a circular lobe, which imparts no lift to the controlvalve and therefore provides valve deactivation.

In the prior art system each valve can be controlled by only twodifferent profiles, a profile for low speed operation and a profile forhigh speed operation. Whilst this is an improvement on the position inconventional engines where one profile is used for all engine speeds andloads, it is preferable to allow more than just two operating conditionsof the cam mechanism, so that the valve motion can better meet therequirements of the engine throughout a range of engine conditions.

In EP-A-0213758 FIGS. 7 to 9 show a cam mechanism for controlling a pairof valves which has three operating conditions. In the first operatingcondition one valve is driven by a finger follower which engages a lowlift cam, whilst the other valve remains deactivated since the fingerfollower that abuts it engages a circular raised portion. In the secondoperating condition both valves are driven by the low lift cam. In thethird operating condition both valves are driven by a high lift cam.Thus one valve has three operating modes, low lift, high lift,deactivated, whilst the other has just two operating modes.

In EP-A-0276533 FIGS. 14 and 15 show a cam mechanism in which a camshafthas a low lift cam, a high lift cam and an annular raised portion. Apair of cylinder head valves are driven by the cam mechanism which hasthree operating conditions; a first in which both valves aredeactivated, a second in which both valves are driven by a low lift camand a third in which both valves are driven by a high lift cam.

In both EP-A-0213758 and EP-A-0276533 three finger followers areprovided, one engaging the circular raised portion, one engaging the lowlift cam and one engaging the high lift cam. An interconnectingmechanism allows two of the finger followers or all three of the fingerfollowers to be connected together and changeover between operatingmodes is achieved in this way. The interconnecting mechanism compriseslocking pins which extend between the finger followers and this requiresgood alignment and precise machining. In the deactivated state of acontrolled valve or both controlled valves the finger followers are heldin engagement with their respective cams or the raised portion bydedicated springs and thus friction losses are incurred in the valvedeactivated state.

It would be difficult to fit hydraulic lash adjusters in either of thesystems of EP-A-0231759 or EP-A-0276533 since it would be difficult toprovide a supply of constant hydraulic pressure to hydraulic lashadjusters mounted in the finger followers because one bore is needed inthe rocker arm for switching pressure.

In U.S. Pat. No. 5,099,806 a cam mechanism is shown which has threefinger followers on a rocker shaft, a first in engagement with acircular portion of a camshaft, a second in engagement with a low liftcam and a third in engagement with a high lift cam. The second and thirdfinger followers can be locked to move with the first finger follower byengagement means. When the second and third finger follower are free tomove with respect to the first finger follower then a valve controlledby the cam mechanism is deactivated. When the second finger follower islocked to the first finger follower and the third finger follower isfree to move with respect to the other two finger followers, then thecontrolled valve is activated and the valve is driven by the low liftcam. When the third finger follower is locked to the first fingerfollower then the valve is activated and is driven by the high lift cam.The engagement means comprises a pair of locking members, one for eachof the second and third finger followers, which can be extended radiallyof the rocker shaft against biasing springs by the action of hydraulicpressure in a bore in the rocker shaft.

It would be difficult to incorporate hydraulic lash adjustment in thesystem of U.S. Pat. No. 5,099,806 since it would be difficult to supplyconstant hydraulic pressure to lash adjusters mounted in the fingerfollowers because the bore in the rocker shaft is needed for the fingerfollower selection pressure. Also frictional losses are incurred duringvalve deactivation since the first, second and third finger followerswill still be in contact with their respective cam lobes (although theseare reduced by the use of a roller follower). The engagement means alsorequires quite precise machining and assembly for the alignment oflocking member with recesses and this alignment must be maintaineddespite wear.

In U.S. Pat. No. 4,475,489 a cam mechanism is shown which has a camshaftwhich has a low lift cam and a high lift cam for each valve. Acontrolled cylinder head valve is engaged by two rocker arms which areeach mounted on one of a pair of movable fulcrums. The movable fulcrumsare alternately displaced to provide the controlled cylinder head valvewith two operating conditions, a first operating condition in which thevalve is controlled by a low lift cam and a second operating conditionin which the valve is controlled by a high lift cam. The two movablefulcrums are moved between extended and retracted positions by twocontrol cams on a control shaft, the two control cams engagingundersurfaces of the fulcrums. The control cams are mounted on a controlshaft. For a multi-cylinder engine, if one control shaft is used thenall the fulcrums would be moved together for all cylinders leading tovalve to cam impacts and clatter. To avoid this problem many separatecontrol shafts would be needed. No valve deactivation is provided by thesystem.

It would be difficult to provide hydraulic lash adjusters in the systemof U.S. Pat. No. 4,475,489 since it would be difficult to provide asupply of hydraulic fluid where needed.

The present invention provides a cam mechanism for controlling themotion of cylinder head valve means of an internal combustion, the cammechanism comprising:

a camshaft having first and second cams which rotate therewith, thefirst and second cams having different cam profiles, and

a drive mechanism operable to relay drive from the first or the secondcam to the controlled cylinder head valve means, the drive mechanismcomprising first and second cam follower means engagable respectivelywith the first and second cams and engagement means for engaging thefirst and second cam follower means with their respective cams, wherein

the first drive mechanism has three operating conditions,

a first operating condition in which the first cam follower means isengaged with the first cam and the valve means is driven by the firstcam,

a second operating condition in which the second cam follower means isengaged with the second cam and the valve means is driven by the secondcam, and

a third operating condition in which both the first and second camfollower means are disengaged from the first and second cams.

The present invention thus provides three operating conditions usingjust two cams.

Preferably the cylinder head valve means is deactivated in the thirdoperating condition.

In the valve deactivated condition there are low or no frictional lossesincurred through cam follower engagement with a surface.

Preferably the first cam follower means comprises a first fingerfollower,

the second cam follower means comprises a second finger follower and

the engagement means comprises first and second movable fulcrum means onwhich the first and second finger followers are respectively rockable,wherein

the first fulcrum means is movable between a first position in which thefirst cam follower means is engaged with the first cam and a secondposition in which the first cam follower means is disengaged from thefirst cam, and the second fulcrum means is movable between a firstposition in which the second cam follower means engages the second camand a second position in which the second cam follower means isdisengaged from the second cam, and wherein

the first fulcrum means is in the first position thereof and the secondfulcrum means is in the second position thereof whilst the drivemechanism is in the first operating condition,

the second fulcrum means is in the first position thereof whilst thedrive mechanism is in the second operating condition, and

both of the first and second fulcrum means are in the second positionsthereof whilst the drive mechanism is in the third operating condition.

Preferably the first fulcrum means is in the first position thereofwhilst the drive mechanism is in the second operating condition.

In a first embodiment the first and second finger followers arepivotally mounted at one end thereof on a slider member which in use isslidably mounted on or in a member fixed in position relative to anengine cylinder head. The slider could be slidable on a post mounted onan engine cylinder head. This has the advantage of compact packaging.Alternatively the slider could be mounted in a slider defined by atubular member connected to the cylinder head either directly or via arocker cover.

Preferably the cylinder head valve means comprises first and secondcylinder head valves, which first and second cylinder head valves in thefirst operating condition are both driven by the first cam, which firstand second cylinder head valves in the second operating condition areboth driven by the second cam and which first and second cylinder headvalves in the third operating condition are both deactivated. Drivingtwo valves (rather than one valve) from a selected one of two differentcams decreases size and aids installation.

In a modification of the first embodiment the first finger follower ispivotally mounted at one end thereof on a first slider member and thesecond finger follower is pivotally mounted at one end thereof on asecond slider member, the first and second slider members being slidableon a post and the first slider member being located on the post beneaththe second slider member whereby when the second cam follower means isin engagement with the second cam means the second slider member abutsthe top of the first slider member and causes the two slider members tomove together.

When the cylinder head valve means comprises first and second cylinderhead valves then preferably when the cam mechanism is in the firstoperating condition the first cylinder head valve is driven by the firstcam and the second cylinder head valve is deactivated, when the cammechanism is in the second operating condition both of the cylinder headvalves are driven by the second cam and when the cam mechanism is in thethird operating condition both of the cylinder head valves aredeactivated.

In the first embodiment the slider member or each slider member ispreferably provided with lash adjustment means which connects the slidermember to the controlled cylinder head valve means. Preferably theslider member or each slider member has a closed bore therein and thelash adjustment means comprises a hydraulic lash adjuster located in theclosed bore, the hydraulic lash adjuster having a portion which abutsthe top of a valve stem of the cylinder head valve means.

In a second embodiment, the first and second followers are pivotallymounted on a frame member, the frame member being pivotally mounted on ashaft and having lash adjustment means which engages the cylinder headvalve means.

In the second embodiment the frame member is preferably pivotallyconnected to the shaft at a first end thereof and is pivotally connectedto the finger followers at the second end thereof remote from the firstend, the finger followers extending from the second end of the framemember towards the first end.

In the second embodiment the frame member preferably has an aperturetherethrough and the finger followers are pivotally connected to theframe for pivotal motion in the aperture.

Preferably the frame member is engagable with a portion of the camshaftspaced from the first and second cams, the frame member engaging thesaid portion of the camshaft when the cam mechanism is in the thirdoperating condition.

The portion of the camshaft engaged by the frame member in the thirdoperating condition could be of circular axial cross-section whereby thevalve means is deactivated in the third operating condition.Alternatively the portion of the camshaft engaged by the frame member inthe third operating condition could have a low lift cam profile.

In a modification of the second embodiment the first finger follower ispivotally mounted on a first frame member and the second finger followeris pivotally mounted on a second frame member, the first and secondframe members being pivotally mounted on a shaft, the first frame memberbeing able to pivot about the shaft relative to second frame member, thefirst frame member engaging a first cylinder head valve of the cylinderhead valve means and the second frame member engaging a second cylinderhead valve of the cylinder head valve means.

Preferably the first frame member is engagable with a first portion ofthe camshaft spaced from the first and second cams, the second framemember is engagable with a second portion of the camshaft spaced fromthe said first portion and spaced from the first and second cams, thefirst and second frame members respectively engaging the first andsecond portions of the camshaft in the third operating condition.

In a third embodiment the first and second finger followers arepivotally mounted to an I-shaped frame member, with one finger followeron each side of an elongate central portion of the I-shaped framemember, the I-shaped frame member being pivotally mounted on a shaft ata first end thereof and housing two separate lash adjuster means at theother end thereof for engaging two cylinder head valves.

Preferably the first and second movable fulcrum means each comprises anabutment member for abutting a finger follower and actuator means formoving the abutment member, the actuator means being operable to moveeach fulcrum means independently of the other fulcrum means. Preferablyeach abutment member comprises a piston movable in a bore provided in acylinder head of an engine or in a housing attached to the cylinderhead, the piston defining a chamber within the bore and the piston beingmovable in response to changes in pressure in the chamber. Preferablythe cam mechanism comprises supply means for supplying hydraulic fluidto the chamber, which supply means includes switching means forswitching the pressure of the hydraulic fluid in the chamber between afirst level and a second higher level.

Preferably the first and second finger followers each have a curvedlower exterior surface which abuts the fulcrum means.

Preferably the first and second finger followers are held in engagementwith the fulcrum means by spring means.

In a second aspect the present invention provides a cam mechanism forcontrolling the motion of cylinder head valve means of the internalcombustion engine, the cam mechanism comprising:

a camshaft having a cam mounted thereon for rotation therewith,

a drive mechanism operable to relay drive from the cam to the controlledcylinder head valve means, the drive mechanism comprising cam followermeans engageable with the cam and engagement means for engaging the camfollower means with the cam, wherein

the cylinder head valve means is activated when the cam follower meansis engaged with the cam by the engagement means, the cylinder head valvemeans being driven by the cam, and

the cylinder head valve means is deactivated when the cam follower meansis disengaged from the cam by the engagement means and the drivemechanism makes no contact with any part of the camshaft.

Preferably the cam follower means comprises a finger follower and theengagement means comprises a movable fulcrum means on which the fingerfollower is rockable, the rockable fulcrum means being movable between afirst position in which the finger follower is engaged with the cam anda second position in which finger follower is disengaged from the cam.

Preferably the finger follower is pivotally mounted on a slider memberwhich is slidably mounted on or in a member fixed in position relativeto a cylinder head, the slider member sliding relative to the memberfixed in position when the finger follower is engaged with the cam.

Preferred embodiments of the present invention will now be describedwith reference to the accompanying drawings in which:

FIG. 1 shows an elevational view of a first embodiment of cam mechanismaccording to the invention, the cam mechanism operating in what will bereferred to as the first operating condition;

FIG. 2 shows an elevational view of the first embodiment of cammechanism illustrated in FIG. 1, the cam mechanism operating in whatwill be referred to as the second operating condition;

FIG. 3 is a cross-sectional elevational view of the first preferredembodiment of cam mechanism shown in FIGS. 1 and 2, the cam mechanismoperating in what will be referred to as the third operating condition;

FIG. 4 is a part cut-away plan view of the first embodiment of cammechanism illustrated in FIGS. 1 to 3;

FIGS. 5a, 6a, 7a are graphs which show the motion of a first controlledvalve respectively in the first, second and third operating conditionsof the cam mechanism, and FIGS. 5b, 6b and 7b show the motion of asecond controlled valve respectively in the first, second and thirdoperating conditions of the cam mechanism;

FIG. 8 shows a part cut-away plan view of a second embodiment of cammechanism according to the invention;

FIG. 9 shows a part cut-away plan view of a third embodiment of cammechanism according to the invention;

FIG. 10 shows in an elevational view a fourth embodiment of cammechanism according to the invention which is a modification of theembodiment of cam mechanism shown in FIGS. 1 to 4;

FIGS. 11 and 12 show part cross-sectional plan view of components of theFIG. 10 embodiment;

FIGS. 13a, 14a, 15a are graphs which show the motion of a firstcontrolled valve respectively in the first, second and third operatingconditions of the cam mechanism of FIG. 10 and FIGS. 13b, 14b, 15b aregraphs which show the motion of a second controlled valve respectivelyin the first, second and third operating conditions of the FIG. 10 cammechanism;

FIG. 16 shows in a plan view a fifth emboidment of cam mechanismaccording to the invention which is a modification of the embodiment ofcam mechanism shown in FIG. 8;

FIG. 17 shows in a plan view a sixth embodiment of cam mechanismsaccording to the invention which is a modification of the embodiment ofcam mechanism shown in FIG. 9;

FIGS. 18a, 19a, 20a are graphs which show the motion of a firstcontrolled valve respectively in the first, second and third operatingconditions of the cam mechanism of FIG. 16 or FIG. 17 and FIGS. 18b, 19band 20b show the motion of a second controlled valve respectively in thefirst, second and third operating conditions of the cam mechanism ofFIG. 16 or FIG. 17.

FIG. 21 is a part cross-sectional elevational view of a seventhembodiment of cam mechanism according to the invention in what will bereferred to as the third operating condition, the cross-section beingtaken along the line B--B shown in FIG. 22;

FIG. 22 is a plan cross-sectional view of the FIG. 21 embodiment, thecross-section being taken along the line A--A shown in FIG. 21;

FIG. 23 is a part cross-sectional view of an eigth embodiment of cammechanism according to the invention, in what will be referred to as thethird operating condition, the cross-section being taken along the lineD-D' shown in FIG. 24;

FIG. 24 is a plan cross-sectional view of the FIG. 23 embodiment, thecross-section being taken along the line C-C' shown in FIG. 23.

Turning first to FIG. 3 there can be seen in FIG. 3 a camshaft 10 havingmounted thereon two cam lobes 11 and 12. The camshaft 10 will beconnected by suitable belt and pulley means to the working crankshaft ofthe engine, so that the camshaft rotates in timed relationship to therotation of the crankshaft.

In FIG. 3 there can be seen a cylinder head valve 13 which would be theinlet or exhaust valve of the internal combustion engine. The cylinderhead 14 of the engine is shown in the FIG. 3 in cross-section.

Fixedly mounted in a bore in the cylinder head 14 is a post 15. Slidableaxially on the post 15 is a slider member 16. The slider member 16 canalso be seen in the plan view of FIG. 4. The top of the valve stem ofcylinder head valve 13 abuts a bucket member 17 of a hydraulic lashadjuster 18 which is provided in a bore in the slider 16. The cylinderhead valve 13 is kept in abutment with the bucket member 17 of thehydraulic lash adjuster 18 by a spring 19 which acts between thecylinder head 14 and a spring retainer 20 which is attached to the valvestem of the valve 13. A circlip 40 is provided on the post 15 to limitupward motion of the slider member 16. The circlip 40 is positioned todefine a base circle position for the slider member 16 (ie. the positionof the slider member 16 when either a finger follower 23 or fingerfollower 24 is in engagement with the base circle portion of a cam; seelater). The provision of the circlip gives the lash adjuster 18 anabutment to act against in a valve deactivated condition (see later).

The cam mechanism of the invention is designed to operate two cylinderhead valves simultaneously. This is illustrated in FIG. 4 where it canbe seen that the hydraulic lash adjuster 18 has a counterpart 21 alsoprovided in a bore in the slider 16.

Situated between the two hydraulic lash adjusters 18 and 21 is a shaft22 which is retained in the slider 16.

Rotatable about the shaft 22 are two finger followers 23 and 24.

Both finger followers 23 and 24 are mirror images of each other andtherefore we will only describe in detail the finger follower 24 whichcan be seen in both FIGS. 3 and 4.

The finger follower 24 comprises a first end portion which has a boretherethrough through which passes the shaft 22. The finger follower 24then has a U-shaped portion at the other end thereof, a shaft 25extending between the two arms of the U-shaped portion of the fingerfollower 24 and a roller follower 26 being mounted on the shaft 25. Thelower portion of the U-shaped part of the finger follower 24 is providedwith a curved exterior surface 27. The exterior surface of the fingerfollower 24 is also provided with an upwardly facing groove 28.

Provided on top of the cylinder block 14 is a housing 29 in which apiston 30 is movable in a bore between an extended uppermost positionand a retracted lowermost position. The finger follower 24 engages thepiston 30, the curved lower exterior surface 27 of the finger follower24 abutting the top of the piston 30. A spring 31 is attached to thehousing 29 by means of a suitable nut 32. The spring 31 engages thegroove 28 in the finger follower 24 and acts between the finger follower24 and the housing 29 to keep the finger follower 24 engaged with thetop surface of the piston 30.

A second piston will be provided in the housing 29, spaced apart frompiston 30. The second piston will be engaged by a lower surface of thefinger follower 23. The second position will also be movable between anextended uppermost position and a retracted lowermost position.

The piston 30 is part of a first fulcrum arrangement for the fingerfollower 24 and the second piston (not shown) is part of a secondfulcrum arrangement for the finger follower 23. Each fulcrum arrangementwill have a latch arrangement for maintaining its piston in itsuppermost position. Latch arrangements are not shown in the figures forthe sake of clarity. Suitable latch arrangements have been shown in thePCT patent publication WO91/12415 and also in the UK patent applicationGB-A-2272022.

In FIG. 3 there can be seen an oil gallery 33 which supplies oil to thebottom face of the piston 30. Also seen in FIG. 3 is an oil gallery 34which will supply oil to the bottom face of the piston 30 which abutsthe finger follower 23.

The hydraulic lash adjusters 18 and 21 in the slider 16 are suppliedwith oil respectively via oil passages 35 and 36 in the slider member 16which are in turn supplied by an oil passage which extends axially ofthe post 15 and which is fed by an oil gallery 37 in the cylinder head14.

The pressure of the oil in the oil galleries 33 and 34 will becontrolled by an electro-mechanical valve (not shown) which in turn iscontrolled by an electronic controller. The electro-mechanical valvewill be able to switch the pressure in each of the oil galleries 33 and34 independently between a low oil pressure and high oil pressure.

In FIG. 3 the third operating condition of the cam mechanism of theinvention is shown and in this operating condition the oil pressure inboth of the oil galleries 33 and 34 is kept low. The oil pressure is notsufficient to force the piston 30 upwardly against the biasing force ofspring 31. Similarly, the oil pressure in the oil gallery 34 is notsufficient to force upwardly the piston which abuts the finger follower23.

In the third operating condition the roller follower 26 of the firstfinger follower 24 does not engage the first cam 11. Also, a secondroller follower 38 provided for the finger follower 23 does not engagethe second cam 12. Since neither cam 11 nor cam 12 are engaged by thefinger followers, no lift is transmitted by the cam mechanism and thevalve 13 remains deactivated. The third operating condition isillustrated by the FIGS. 7a and 7b.

FIG. 7a shows that no lift is applied to the valve 13 when the cammechanism is in the first operating condition. FIG. 7b similarly showsthat no lift is applied to the other valve of the controlled pair ofvalves when the cam mechanism is in the third operating condition.

In the third operating condition the slider member 16 is held inabutment with the circlip 40.

When the oil pressure in oil gallery 33 is switched to high oilpressure, whilst the oil pressure in gallery 34 is kept low, the cammechanism assumes the first operating condition shown in FIG. 1. In FIG.1, the finger follower 23 can be clearly seen in elevation as can aspring 39 which acts between the housing 29 and the finger follower 23to keep the finger follower 23 engaged with a top surface of a piston(not shown) which corresponds to piston 30.

The oil pressure in gallery 33 is sufficient to move the piston 30against the biasing force of spring 31 to a raised position in which theroller follower 26 engages the profile of the cam 11. The piston 30 willbe locked in this position by a latching mechanism (not shown). It willbe seen that the finger follower 24 has in fact pivotted about the shaft22 to a position in which the roller follower 26 engages the cam 11.

In the first operating condition shown in FIG. 1 a finger follower 24pivots about the piston 30 under the action of the cam 11. This isallowed by the curved nature of the lower exterior surface 27 of thefirst finger follower 24. The pivotting of the finger follower 24 underthe control of the cam 11 causes the slider 16 to move reciprocably upand down the post 15, against the biasing force of the spring 19. Thisin turn causes motion of the valve 13 and activates the valve. The valve13 in this operating condition follows the profile of low lift cam 11.

FIG. 5a shows the low and short duration lift applied to the valve 13when the cam mechanism is in the second operating condition. FIG. 5bshows that identical lift is applied to the other of the pair of valves.

The second operating condition of the first embodiment of cam mechanismof the invention is shown in FIG. 2. In the FIG. 2 operating conditionthe pressure of the oil in both of the galleries 33 and 34 will be kepthigh and thus the fulcrum means for roller follower 38 will be locatedin its uppermost position by latching means and the roller follower 38engages the cam 12, whilst the roller follower 26 still engages the cam11. The finger follower 24 will pivot about the piston 30 and a fingerfollower 23 will pivot about the piston engaged thereby.

In the FIG. 2 operating condition, the slider 16 is caused toreciprocally move up and down the post 15 by the action of the cam 12.Since the cam 12 is a high lift cam and the cam profile of cam 12completly encompasses the cam profile of cam 11, the motion of theslider 16 and thus the motion of the valve 13 is controlled by the highlift cam 12.

FIG. 6a shows the high and long duration lift applied to the valve 13when the cam mechanism is in the second operating condition. FIG. 6bshows that identical lift is applied to the other of the pair of valves.

It is envisaged that the cam mechanism of the invention could be usedfor two valves of an engine which has four cylinder head valves percylinder.

For a four valve per cylinder engine one cam mechanism could controlboth inlet valves of one cylinder and another cam mechanism couldcontrol both exhaust valves of the cylinder.

When the cam mechanisms are in the deactivated state both inlet and/orboth exhaust valves would be deactivated. This would prevent any flow ofair or fuel through the cylinder, hence deactivating the cylinder. Thiswould be done for say two cylinders of a four cylinder engine or fourcylinders of a six or eight cylinder engine, at low load and speedconditions. The object of this is to make the remaining workingcylinders work harder and therefore more efficiently thus improving theoverall engine efficiency.

In a mid-range condition of the engine, for mid-range speeds and loads,the cam mechanism will assume the operating condition shown in FIG. 1.In this operating condition, the pair of valves are activated andcontrolled by the profile of the cam 11, which is a cam suited to lowspeed and/or load operation.

As the engine speed and/or load increases, the electronic controller maybring the cam mechanism to the operating condition shown in FIG. 2, inwhich the pair of valves are activated and controlled by the profile ofthe cam 12. The cam 12 has a high lift profile and one which is suitedto high speed and/or high load operation of the engine.

It will be appreciated that the first embodiment is very compact innature, since it requires only two arms in the form of the fingerfollowers 23 and 24 and the packaging can be made quite compact. It isimportant to achieve compact packaging since space is limited at thecylinder head of any engine. Also, compact packaging can lead to asaving in the mass of the valve train which cuts down losses.

The second embodiment of cam mechanism according to the presentinvention is shown in FIG. 8. In FIG. 8 the finger followers 50 and 51are identical to the finger followers 23 and 24, the finger followers 50and 51 having roller followers 52 and 53 respectively.

Both of the finger followers 50 and 51 are rotatable about a shaft 54,in a similar manner to the first embodiment. However, the shaft 54 isnot positioned in a slider, but is positioned in a D-shaped frame member55 which is pivotally mounted on a shaft 56. The finger followers 50 and51 and the roller followers 52, 53 pivot relative to the D-shaped framemember 55 through the aperture in the frame member 55. Two hydrauliclash adjusters 57 and 58 are provided in bores in the frame member 55and each contact the top of the valve stem of a cylinder head valve. TheD-shaped frame member 55 will at two spaced apart points abut circularcross-section portions of a camshaft located thereabove.

The second embodiment of the cam mechanism works in the same way as thefirst described embodiment. The finger followers 50 and 51 will each bein contact with one of two movable fulcrums and three operatingpositions of the cam mechanism are possible. With both fulcrums in theirlower position, the roller followers 52 and 53 are held out ofengagement with the cams of a camshaft located thereabove. The framemember 55 will abut the circular portions of the cam but these willimpart no lift to the valves and thus the controlled valves remaininactive. The hydraulic lash adjusters 57 and 58 will not overextend inthe valve deactivated condition due to the engagement of the framemember 55 with circular portions of the camshaft. The frictional losseswill not be great in the valve deactivated position since the only forceacting to force the frame member against the rotating camshaft is thebiasing force from weak springs in the hydraulic lash adjusters. Ifmechanical lash adjustment in the form of shims is used then there willbe nearly zero frictional losses.

When the finger follower 51 is raised by the fulcrum means it engagesthen the roller follower 53 with a low lift cam and motion of the lowlift cam is transmitted via the roller follower 53 and the fingerfollower 51 to the frame 55 and thus to both controlled engine valves.

When both the finger follower 51 and the finger follower 50 are raisedby the movable fulcrums beneath then the roller follower 52 will engagea high lift cam. Since the profile of the high lift cam encompasses theprofile of the low lift cam, the frame 55 will be moved under the motionof the high lift cam and will cause motion of the two controlledcylinder head valves.

The second embodiment shown in FIG. 8 is not as compact as the firstembodiment shown earlier and requires more room at the cylinder head formounting. Also, the mass of the cam mechanism is higher than the mass ofthe first embodiment and this is disadvantageous. Nevertheless, thesecond embodiment will be used in certain circumstances in preference tothe first embodiment, where the mounting of a post 15 with associatedslider is not possible.

A third embodiment of cam mechanism according to the present inventionis shown in FIG. 9. In common with the FIG. 8 embodiment, the FIG. 9embodiment has a frame 60 rotatable about a shaft 61. The frame has twobores 62 and 63 which receive in use hydraulic lash adjusters which inturn engage the tops of the valve stems of two cylinder head enginevalves.

The frame 60 of FIG. 9 is a generally I-shaped. The finger followers 64and 65 of the FIG. 9 embodiment are not located side by side as in theearlier embodiments, but are spaced apart. Finger followers 64 and 65are mounted on the outer ends of a shaft 68 which extends through theframe member 60. The central portion of the I-shaped frame 60 will beengageable with a circular portion of a camshaft mounted thereabove (notshown).

The finger followers 64 and 65 respectively have roller followers 66 and67. Each finger follower 64 and 65 is mounted on one of two movablefulcrums (not shown).

When both of the movable fulcrums are in their lower position, neitherroller follower 66 nor roller follower 67 engages a cam of a camshaft.The frame 60 is in engagement with the circular cross-section portion ofthe camshaft which imparts no lift to the frame 60 and therefore nomotion is transmitted to the two controlled cylinder head engine valves;the valves are deactivated.

When the finger follower 65 is raised by the fulcrum beneath then theroller follower 67 is brought into engagement with a low lift cam on acamshaft (not shown). The motion of the low lift cam is then transmittedthrough the roller follower 67 and finger follower 65 to the shaft 68and then to the frame 60. The motion is then relayed through thehydraulic lash adjusters in the bores 62 and 63 to the two cylinder headengine valves, which are controlled by the profile of the low lift camengaged by roller follower 67.

When the fulcrum under finger follower 64 is subsequently raised, theroller follower 66 is brought into engagement with a high lift cam andthe motion of the high lift cam is transmitted via the roller follower66, finger follower 64, shaft 68 and frame 60 to the hydraulic lashadjusters 62 and 63 and then to the two controlled engine valves. Theprofile of the high lift cam will be chosen to completely envelope theprofile of the low lift cam, so that when both fulcrums are raised themotion of the two controlled cylinder head valves is controlled only bythe high lift cam.

The FIG. 9 embodiment will be used when the cams on the camshaft arefurther spaced apart than in the embodiments of FIGS. 1 to 8. However,the arrangement is not as compact as the embodiment of FIGS. 1 to 4 andwill be of greater mass.

When the cam mechanism of the third embodiment is in the valvedeactivated operating condition frictional losses are reduced to aminimum by having the finger followers 64 out of engagement with thecams. Some frictional losses result from engagement of the frame member60 with the camshaft, but the only biasing force in the frame member inthe valve deactivated state results from the small spring force exertedby the springs in the hydraulic lash adjusters (if mechanical lashadjustment was provided by shims then even this force would not bepresent). The abutment of the frame member 60 with the camshaft in thevalve deactivated condition is necessary to prevent the hydraulic lashadjusters overextending.

FIG. 10 shows how the embodiment illustrated in FIGS. 1 to 4 can bemodified to provide greater flexibility. In FIG. 10, the majority of thecomponents are identical to the components already described in FIGS. 1to 4 and accordingly identical reference numerals have been used.

The difference between the embodiment shown in FIG. 10 and theembodiment shown in FIGS. 1 to 4 lies in the fact that the embodiment ofFIG. 10 has two slider members 70 and 71, rather than one slider memberas in the embodiment shown in FIGS. 1 to 4.

Two slider member 70 and 71 are both slidable on the same post 15 andhave aligned bores therethrough. The slider member 70 is shown in planpart cross-section view in FIG. 11 and in the figure it can be seen thatthe finger follower 23 is pivotally connected to the slider member 70 bya shaft 72. The finger follower 23 is connected only to the slidermember 70 and is not in any way pivotally connected to the other slidermember 71.

The slider member 70 has a hydraulic lash adjuster 73 for abutting thetop of a first controlled cylinder head valve of a pair of cylinder headvalves. The hydraulic lash adjuster is supplied with oil through apassage 74.

In FIG. 12 a plan part cross-section view of the lower slider member 71can be seen and in the FIG. 12 it can be seen that the finger follower24 is pivotally connected to the lower slider member 71 and is not inany way pivotally connected to the upper slider member 70. The fingerfollower 24 is connected to the lower slider member 71 by a shaft 75. Ahydraulic lash adjuster 76 is provided in the slider member 71 and thehydraulic lash adjuster is supplied by a passage 77. The hydraulic lashadjuster 76 will abut the valve stem of the second cylinder head valveof the pair of cylinder head valves.

Since the embodiment has two slider members, the modes of operation ofthe mechanism are different to the embodiments shown in FIGS. 1 to 4.The modes of operation are illustrated in FIGS. 13a, 13b, 14a, 14b, 15aand 15b.

Turning first to FIGS. 13a and 13b, these figures illustrate theoperating condition actually shown in FIG. 10. In FIG. 10 the movablefulcrum 30 is raised to engage the roller follower 26 with the camsurface 11. The slider member 71 will thus move reciprocally up and downthe post 15, under the control of the cam 11. The cam 11 is a low liftcam. The motion of the valve controlled by slider member 71 can be seenin FIG. 13a in this operating condition.

In the same FIG. 10 operating condition, the other movable fulcrum iskept in its lowermost position so that no motion is tranmitted from thecam 12 to the slider member 70. Consequently, the valve controlled bythe slider member 70 remains inactive as can be seen in FIG. 13b.

When the roller follower 38 mounted on finger follower 23 is brought upinto engagement with the cam 12 by means of the movable fulcrum situatedthere beneath, the slider member 70 is caused to move by the action ofthe cam 12. Since the slider member 70 is positioned above the slidermember 71, the slider member 70 causes motion also of the slider member71. Since the lift of the high lift cam 12 is greater than andcompletely encompasses the lift of the lower lift cam 11, both slidermembers 70 and 71 follow the profile of the high lift cam 12 and henceboth of the controlled cylinder head valves are given high lift. Thiscan be seen clearly in FIGS. 14a and 14b where the valve motion of bothvalves is shown.

When both of the roller followers 26 and 38 are brought out of contactwith their respective cams then no motion is transmitted to either ofthe controlled valves and this is illustrated in FIGS. 15a and 15b.

A modification of the FIG. 8 embodiment is shown in FIG. 16. Themajority of the components in both embodiments are identical andtherefore we will not describe in detail the components of FIG. 16.

The significant difference between the FIG. 16 and FIG. 8 embodiments isthe provision in the FIG. 16 embodiment of a frame member whichcomprises two halves, 155a and 155b. Also, the shaft 54 in the FIG. 8embodiment is replaced by two shafts 154a and 154b. In this way, theframe half 155b can move independently of the frame half 155a andtherefore each valve of a controlled pair of valves can have a differentmotion to the other. However, a bridging member 170 is attached to theframe half 155a which can abut the top surface of the frame half 155b.Thus when the frame half 155a is displaced downwardly by a cam more thanframe half 155b then the bridging member 170 abuts top surface of framehalf 155b and both frame half 155a and frame half 155b move together.

In a similar manner, FIG. 17 shows a modification of the FIG. 9embodiment, with the I-shaped frame 60 of the FIG. 9 embodiment replacedby a split frame comprising two halves 160a and 160b. The shaft 68 isreplaced by two shafts 168a and 168b. In this way, frame half 160b canmove independently of frame half 160a so that each of a controlled pairof valves can move with a different motion to the other. However, abridging portion 169 is attached to the frame half 160a which can abutthe frame half 160b so that the two halves move together when frame half160a is displaced more than frame half 160b.

Each of the frame halves 155a and 155b will engage separate portions ofa camshaft. When both movable fulcrums are in their lowermost positionand the roller followers 152 and 153 are out of engagement with theircams, then the cylinder head valve abutting lash adjuster 157 will becontrolled by the portion of the camshaft abutted by the frame half155a. Similarly, the cylinder head valve abutted by hydraulic lashadjuster 158 will be controlled by the portion of the camshaft abuttedby frame portion 155b. In a similar manner, frame portions 160a and 160bwill also abut two different portions of a camshaft and the valvescontrolled thereby will be controlled in accordance with the shape ofthe camshaft portions abutted by the two frame halves 160a and 160b.

The camshaft portions abutted by the frame members could be circular innature, so that the valves controlled have no lift (as illustratedbefore). Alternatively, the portions of the camshaft abutted by theframe halves could have some lift and indeed could have lift differentto each other.

FIGS. 18a, 18b, 19a, 19b, 20a and 20b show valve motion for either ofthe embodiments of FIG. 16 or FIG. 17, when the frame halves of eitherembodiment abut portions of the camshaft which have differing lifts.

FIGS. 18a and 18b show an operating condition in which a low lift cam isengaged (either by roller follower 153 or roller follower 167, dependingon the embodiment), whilst the other roller follower (either 152 or 166)remains out of engagement with the high lift cam. It can be seen fromFIG. 18a that one controlled valve follows a low lift cam profile. Itcan be seen from FIG. 18b that the other controlled valve follows thevery low lift profile of the portion of the camshaft abutted by theframe member 155a or 160a.

In FIGS. 19a and 19b there is shown a condition in which a high lift camis abutted (either by roller follower 152 or roller follower 166). Inthis case, both controlled valves follow the high lift cam profile, thevalve half 155a engaging the valve half 155b by means of the bridgingmember 170 or the frame half 160a engaging the upper surface of theframe member 160b by means of the bridging member 169.

In the operating condition of FIGS. 20a and 20b the low lift and highlift cams provided on the camshaft are not engaged by either of theroller followers and the two frame halves are running on the camshaftand one valve is controlled by the profile of the portion of thecamshaft abutted by the frame members 155b and 160b (as shown in FIG.20a), whilst the other controlled valve is controlled by the cam profileof the portion of the camshaft abutted by the frame members 155a or 160a(as shown in FIG. 20b).

FIGS. 21 and 22 show a further embodiment of cam mechanism according tothe invention. The principle of operation of the mechanism is the sameas described above for the earlier embodiments, but the packaging isdifferent. Many components are also identical with earlier describedcomponents and will be given identical reference numerals. We shallconcentrate on the differences between the embodiment of FIGS. 21 and 22and earlier embodiments.

In FIG. 21 the finger followers 300 and 301 are similar to fingerfollowers 23 and 24 in that they have a bore through which a shaft 302passes and each have a U-shaped section. In between the arms of theU-shaped section of finger follower 300 a roller follower 303 is mountedon a shaft 304 which is secured in the arms. In between the arms of theU-shaped section of finger follower 301 a roller follower 305 is mountedon a shaft 306 which is secured in the arms. The finger follower 300 hasa curved lower face 307 for abutting the top of the piston 30 and thefinger follower 301 has a similar surface (not shown) for abutting thetop of a further piston 310. The spring 31 locates in a groove 322 onthe top of finger follower 300 and a similar spring (not shown) willlocate in a groove 320 in the top of finger follower 301, the springsacting to hold the finger followers 300 and 301 in abutment with thepistons 30 and 310.

The two finger followers 300 and 301 abut along facing side but are freeto move relative to each; the shafts 304 and 306 are not joined.

The main difference between the embodiment of FIGS. 21 and 22 and theembodiment of FIGS. 1-4 lies in the fact that in the FIGS. 21 and 22embodiment the finger followers 300 and 301 are mounted to pivot on ashaft 302 which spans between two sliding members 307 and 308 which areslidable inside two cylinders defined at the cylinder head of the engineby tubular members 309 and 310. The tubular members 309 and 310 can, forinstance, be formed either as part of the rocker cover of the engine orare preferably formed integrally with the cylinder head. The tubularmembers 309 and 310 each define a cylinder partially closed at one endwith outlet passages (not shown) allowing communication of each cylinderwith the exterior of the tubular members 309 and 310 in order to preventa hydraulic lock forming (alternatively the tubular members could eachhave an open bore extending therethrough with circlips acting as a stopfor the slider members within the bores). In each cylinder there islocated one of the sliding members 307 or 308. Each sliding member 307or 308 has a closed bore in which a hydrualic lash adjuster such as 18(see FIG. 21) is located. Each hydraulic lash adjuster abuts the top ofa stem of controlled valve (e.g. lash adjuster 18 abuts the top of thevalve 13). The hydraulic lash adjusters will be supplied with oil viapassages in the sliding members 307 or 308 and passages in the tubularmembers 309 and 310; e.g. the lash adjuster 18 is supplied with oil viaa passage 311 in the sliding member 309 which is in turn supplied withoil from a gallery 312 in the tubular member 309.

The embodiment of FIGS. 21 and 22 works in the same way as theembodiment of FIGS. 1 to 4. If both roller followers 303 and 305 are outof contact with cams 11 and 12 then the two controlled valves (only 13shown) remain deactivated. If the roller follower 303 contacts cam 11whilst the roller follower 305 remains out of contact then both thecontrolled valves receive low lift from the cam 11 via the rollerfollower 303, the finger follower 300, the shaft 302 and the slidingmembers 307 and 308 (with their respective lack adjusters). If bothroller followers 303 and 305 are in the raised positions then bothcontrolled valves will be controlled by the high lift of cam 12transmitted through the roller follower 305, the finger follower 301 andthe sliding members 307 and 308 (with their respective lash adjusters).The partially closed ends of the cylinders in the tubular members 309and 310 define end stops for the sliding members 307 and 308 and givethe lash adjusters references to work from (alternatively circlips inopen bores could provide the references).

The embodiment of FIGS. 23 and 24 is similar in many ways to the earlierembodiments and identical components will be given identical referencenumerals.

The finger followers 400 and 401 of the embodiment of FIGS. 23 and 24are each identical and are each U-shaped. The finger followers 400 and401 are both pivotally mounted on a U-shaped frame 403 which ispivotally mounted on a shaft 402, with the arms of the U-shaped frameextending away from the shaft 402 towards the controlled valves (e.g.13). The U-shaped frame 403 is symetrically split in a directiontransverse of the shaft 402 into two halves 403A and 403B, each halfbeing pivotal about the shaft 402 independently of the other. One arm404 of the U-shaped frame 403 is interposed between two arms of fingerfollower 400 and the other arm 405 is interposed between the two arms ofthe finger follower 401. A shaft 406 extends through the arm 404 of theU shaped member and the two arms of the finger follower 400 to link themall together. A shaft 407 extends through the arm 405 of the U shapedmember and the two arms of the finger follower 401 to link them alltogether.

Two roller followers 408 and 409 are rotatably mounted on the fingerfollower 400, on the exterior sides of the finger follower 400, two halfshafts 410 and 411 being rotatably mounted one in each of the arms ofthe finger follower 400, extending outwardly. In a similar fashion tworoller followers 442 and 443 are rotatably mounted on the fingerfollower 401, on the exterior sides of the finger follower 401, two halfshafts 444 and 445 being rotatably mounted one in each of the arms ofthe finger follower 401 extending outwardly.

The arms 404 and 405 of the U-shaped frame 403 are each provided with acentral cut out portion in which a roller follower is mounted on a shaftextending through the arm (roller follower 412 is mounted on a shaft 414in the arm 404 and roller follower 433 is mounted on a shaft 415 in thearm 405).

The arm 404 is pivotal about a shaft 416 at the end of the arm furthestdistance from shaft 402. The shaft 416 is mounted on a slider member 417slidable in an open bore in a tubular member 418 which is either part ofthe rocker cover or preferably part of the cylinder head. In the slidermember 417 a lash adjuster 18 is provided which abuts the top of thecontrolled valve 13. The shaft 416 is slidable in a slot 470 provided inthe arm 404, the slot 470 being necessary to allow relative motionbetween the arm 404 and shaft 416 when the arm 404 pivots about shaft402.

The arm 405 is pivoted about a shaft 419 at the end of the arm 404furthest distance from the shaft 402. The shaft 419 is mounted in aslider member 420 which is slidable in an open bore defined in a tubularmember 421 which is either part of the rocker cover or preferably partof the cylinder head. In the slider member 420 a lash adjuster (notshown) is provided which abuts the top of the stem of a controlled valve(not shown). The shaft 419 is slidable in a slot 471 (see FIG. 24)provided in the arm 405, the slot 471 being necessary to allow relativemotion between the arm 405 and the shaft 419 when the arm 405 pivotsabout shaft 402.

The finger follower 400 has a curved lower surface 422 which abuts thetop of the piston 30 and which allows the finger follower 400 to pivotabout the top of the piston 30. The finger follower 400 is held inabutment with the piston 30 by the spring 31 which engages a groove 423provided in the upper surface of the finger follower 400. In a similarfashion the other finger follower 401 has a curved lowermost surface(not shown) which abuts the top of a piston (not shown) and the fingerfollower 401 is held in engagement with the other piston by a spring(not shown) engaged with a groove (not shown) in the top surface of thefinger follower 401.

When both of the pistons in the cylinder head are in their lowestpositions the roller followers 408 and 409 of the finger follower 400and the roller followers 443 and 442 of finger follower 401 are all outof engagement with the cams on the camshaft 10. The roller followers 412and 433 mounted in the arms 403 and 404 of frame 403 engage the camshaft10. If the roller followers 412 and 413 engage a circular portion of thecamshaft 10 then the slider members 417 and 420 remain still and thecontrolled valves (e.g. 13) inactive. However, if the roller followers412 and 433 engage cams on the camshaft 10 then the arms 404 and 405will relay motion to the slider members 412 and 420 which will slide andcause valve motion. Since the U-shaped frame 403 is split into twohalves (403A and 403B) the arm 404 can engage a cam of different lift tothe cam engaged by arm 405 and thus arm 404 can move relative to are 405so that the arms 405 and 404 transmit different lifts to the twocontrolled valves.

When the piston 30 is raised the roller followers 408 and 409 of thefollower 400 are brought into engagement with two cams of indenticalprofile on the camshaft 10. If the other piston remains lowered then thevalve 13 will be controlled by the lift of the cams engaged by thefollower followers 408 and 409, whilst the valve controlled by thepiston remains under control of the roller follower 433 following acircular piston of the camshaft 10 or a cam of low lift.

When the piston 30 is raised and the other piston is raised then afurther mode of operation is occasioned. A dog clutch 473 is provided asa cross=linking arrangement which does not allow the arm 405 to movedownwardly further than arm 404 by acting between the two halves 403Aand 403B of the frame member 403 where they abut on the shaft 402.Alternatively cross linking could be provided by a bar extending acrossfrom arm 405 to abut the top surface of arm 404. When the rollerfollowers 442 and 443 of the arm 405 engage cams of higher lift than thecams engaged by the roller followers 408 and 409 of arm 404 then thecontrolled valves will both be given the same lift, a lift dictated bythe higher lift cams engaged by the roller followers 442 and 443.

The lash adjuster 18 and the other lash adjuster (not shown) will bothbe supplied by oil through the tubular members 418 and 421 and theslider members 417 and 420. The lash adjusters use as a reference foradjustment the abutment of the roller followers 412 and 433 with the camshaft 10.

The use of slider members which slide inside a cylinder (see FIGS. 21 to24) rather than on posts (see FIGS. 1 to 4) has the advantage of apotentially lighter reciprocating mass. Also machining of bores to formthe cylinders is a relatively simple machining operation which can bedone when the bores for the valves are machined in the cylinder headachieving instant alignment between the sliders and the controlledvalves.

All of the above embodiments share very important advantages in thatthree operating conditions are provided with just two movable fulcrumsand in that neither of the roller followers contact their respectivecams in the valve deactivated operating condition. Thus no valve trainfriction is incurred at all and this reduces engine losses.

We claim:
 1. A cam mechanism for controlling the motion of cylinder headvalve means of an internal combustion engine, the cam mechanismcomprising:a camshaft having first and second cams which rotatetherewith, the first having a cam profile which imparts a first lift andthe second cam having a cam profile which imparts a second higher lift,and a drive mechanism operable to relay drive from the first or thesecond cam to the controlled cylinder head valve means, the drivemechanism comprising first and second cam follower means engageablerespectively with the first and second cams and engagement means forengaging the first and second cam follower means with their respectivecams, the cylinder head valve means further comprising first and secondcylinder head valves, said drive mechanism having three operatingconditions;(a) a first operating condition in which the first camfollower means is engaged with the first cam and the first cylinder headvalve is driven by the first cam, (b) a second operating condition inwhich the second cam follower means is engaged with the second cam andthe first and second cylinder head valves are both driven by the secondcam; and (c) a third operating condition in which both the first andsecond cam follower means are disengaged from the first and second cams,wherein in the first operating condition, the first cylinder head valvereceives a lift different from the lift received by the second cylinderhead valve.
 2. A cam mechanism as claimed in claim 1 wherein the firstand second cylinder head valves are both deactivated in the thirdoperating condition.
 3. A cam mechanism as claimed in claim 1 whereinthe second cylinder head valve is deactivated in the first operatingcondition.
 4. A cam mechanism as claimed in claim 1 in which the firstcam follower means comprises a first finger follower,the second camfollower means comprises a second finger follower, and the engagementmeans comprises first and second movable fulcrum means on which thefirst and second finger followers are respectively rockable, wherein thefirst fulcrum means is movable between a first position in which thefirst cam follower means is engaged with the first cam and a secondposition in which the cam follower means is disengaged from the firstcam, and the second fulcrum means is movable between a first position inwhich the second cam follower means engages the second cam and a secondposition in which the second cam follower means is disengaged from thesecond cam, and wherein the first fulcrum means is in the first positionthereof and the second fulcrum means is in the second position thereofwhile the drive mechanism is in the first operating condition, thesecond fulcrum means is in the first position thereof while the drivemechanism is in the second operating condition, and both of the firstand second fulcrum means are in the second positions thereof while thedrive mechanism is in the third operating condition.
 5. A cam mechanismas claimed in claim 4 wherein the first fulcrum means is in the firstposition thereof while the drive mechanism is in the second operatingcondition.
 6. A cam mechanism as claimed in claim 4 wherein the firstand second finger followers are respectively pivotally mounted each atone end thereof on first and second slider members which in use areslidably mounted on or in a member fixed in position relative to anengine cylinder head.
 7. A cam mechanism as claimed in claim 6 whereinboth the first and second slider members are slidable on one postmounted on the cylinder head.
 8. A cam mechanism as claimed in claim 6wherein each of the first and second slider members is slidable in acylinder defined in a tubular member connected to the cylinder headdirectly or via a rocker cover.
 9. A cam mechanism as claimed in claim 7wherein the first slider member is located on the post beneath thesecond slider member whereby when the second cam follower means is inengagement with the second cam means the second slider member abuts thetop of the first slider member and causes the two slider members to movetogether.
 10. A cam mechanism as claimed in claim 6, wherein each slidermember is provided with lash adjustment means which connects the slidermember to at least one of controlled cylinder head valves.
 11. A cammechanism as claimed in claim 10 wherein each slider member has a closedbore therein and the lash adjuster means comprises a hydraulic lashadjuster located in the closed bore, the hydraulic lash adjuster havinga portion which abuts the top of a valve stem of one of the cylinderhead valves.
 12. A cam mechanism as claimed in claim 4 wherein the firstand second movable fulcrum means each comprises an abutment member forabutting a finger follower and actuator means for moving the abutmentmember, the actuator means being operable to move each fulcrum meansindependently of the other fulcrum means.
 13. A cam mechanism as claimedin claim 12 wherein each abutment member comprises a piston movable in abore provided in a cylinder head of an engine or in a housing attachedto the cylinder head, the piston defining a chamber within the bore andthe piston being movable in response to changes in pressure in thechamber.
 14. A cam mechanism as claimed in claim 4 wherein the first andsecond finger followers each have a curved lower exterior surface whichabuts the fulcrum means.
 15. A cam mechanism as claimed in claim 4wherein the first and second finger followers are held in engagementwith the fulcrum means by spring means.